66 Shingle on the East Coasts of New Zealand. 



the shore, and carry towards the shore material in suspension 

 near the surface. In fine and calm weather there would be 

 less to obstruct the motion seawards of particles of matter 

 held in suspension near the surface. 



33. Even when the waves are not removing material from 

 one part of the slope to another, the bed of the sea near the 

 shore — that is, in shallow water — must be in a state of per- 

 petual motion, unless the sea is too quiet to stir the material 

 of which the bed is composed. The finer the material the 

 greater the depth of water in which it would be moved. 

 Coarser material at a considerable depth and heavy material 

 near the shore might not be similarly disturbed except by a 

 heavy sea. The author has observed this action on a small 

 scale through clear water. The sand, a little way from the 

 water-line, was continually oscillating up and down the 

 slope — towards the shore as each wave came in, and away 

 from it as the wave returned. With large waves the motion 

 would be similar at a greater depth. It is almost certain 

 that, at times, this disturbance takes place at a very 

 considerable depth. 



fd.) Relation between Wave-action and the Slope of a 



Beach. 



34. The effect of the slope on the waves, or the action of 

 waves on different slopes, may now be considered. Not only 

 boulders and shingle, but sand, merely, is sufficient, under 

 certain conditions, to withstand the force of the waves, even 

 in the heaviest sea. It will, however, be observed that 

 where the seashore consists of sand, there is a very flat slope ; 

 where the material is shingle or small stones, under similar 

 conditions, the slope is greater ; and where there are large 

 stones or boulders, the slope may be very steep. It will also 

 be observed that below the water-line in each case, as a 

 rule, the slope gradually becomes less and less ; and further, 

 where there is shingle on the beach, smaller and finer 

 material is usually found below the water-line, sometimes 

 sand only, and not shingle, is to be found below low-water 

 level. 



35. It would seem that, as shingle or sand is carried alter- 

 nately up and down the slope, and boulders also in a heavy 

 sea, the normal slope in either case is that which allows the 

 usual waves so to break as to move the material to the same 

 extent both up and down. If a little material in excess is 

 moved upwards, at one time, the irregularity is corrected by 



